150 Mr B. Martin's Experiments on Island Crystal, 
Sect III. — Account of the Experiments of Mr Benjamin 
Martin. 
The following Pamphlet, which is published without a date, 
is entitled, An Essay on the Nature and wonderful Properties 
cf Island Crystal , respecting its manifold and unusual R frac- 
tion of Light. By Benjamin Martin. 
“ Island Crystal is a subject of so singular a nature, that it 
has attracted the attention, and excited the curiosity and admi- 
ration of all mankind since its discovery by Erasmus Rartholine , 
about 100 years ago, who was the first that gave any account 
of the figure and properties thereof to the public. 
a The great Hugenius soon after published his treatise on 
light (De la Lumiere), and gave us therein a more correct and 
particular account of this substance, in regard to its peculiar 
forms, dimensions, and strange refraction of light. And because 
that book is now rarely to be met with, and no extract having 
been made from it (but a small sketch in Sir Isaac Newton’s op- 
tics) that I know of, I have thought my spare hours could not 
be better employed, than in translating so much of that valuable 
work as immediately relates to the subject of Island Crystal ; 
especially as I shall thereby have an opportunity of adding se- 
veral new experiments and observations, concerning its wonder- 
ful properties of refracting light. 
“ This species of crystal was originally brought from Island (or 
Ice-land), an Isle in the Northern Sea, about Latitude 65 de- 
grees, and from whence it took its name ; but it has since been 
found in many other parts of the world, and very plentifully in 
Great Britain, particularly in the peak of Derbyshire, though the 
greatest part is not very transparent. 
“ The form of this crystal or talk, is that of an Oblique Par - 
allelopiped , as represented in Plate III. Fig. 1. by ACBDEKFL, 
with six parallelogram sides, and eight solid angles. Two of 
these solid angles opposite to one another, as C and E, are com- 
passed each of them by three equal obtuse angles, each equal to 
101 degrees and 5& minutes ; and the other six solid angles, A, 
L, F, D, B, K, are each contained under one obtuse angle, and 
two acute ones of 78° 8' each. 
